1. Field of the Invention
The present invention relates to braking systems for heavy duty vehicles, and in particular to an improved diaphragm-based spring brake actuator which provides significantly increased braking force from a spring brake assembly having a size that is the same as or smaller than existing brake assemblies.
2. Description of the Prior Art
Various forms of pneumatic vehicle spring brake actuators have been introduced over the years primarily for use in the trucking industry. A typical double diaphragm air brake actuator includes two portions: an operator controlled service brake portion which is used for slowing or stopping a vehicle, and an emergency or parking brake portion which automatically engages when air pressure is removed. A typical service brake portion is characterized by a closed housing which contains a movable diaphragm stretched across the inside. One side of the diaphragm is closely associated with a centrally located pressure plate attached to a slidable push rod which extends out of the housing for attachment to the brakes of the vehicle. On the other side of the diaphragm a sealed chamber is formed within the housing.
An opening is provided in the sealed service brake chamber for connection to a pneumatic (air) pressure source usually provided by an on-board air compressor. The brakes of the vehicle can be applied by introducing sufficient pneumatic pressure into the sealed chamber to act against the service brake diaphragm which moves the plate, pushing the push rod out. A small return spring is ordinarily provided inside the service brake housing around the push rod and adjacent to the pressure plate to urge it to retract when the air pressure behind the diaphragm is reduced.
A typical emergency brake portion of an air brake actuator is attached in axial alignment with or made a part of the service brake assembly. The emergency brake is a separate closed housing which contains a heavy main compression spring and a second movable diaphragm creating a second sealed chamber. The emergency brake diaphragm is in contact with a second pressure plate which is also attached to or directly associated with the slidable central push rod of the service brake.
The second sealed chamber is formed inside the emergency brake housing on one side of the diaphragm, and the heavy main compression spring is deployed on the opposite side. As with the service brake, the sealed chamber of the emergency brake is connected to the on-board pneumatic source of the vehicle. As long as sufficient air pressure is provided to the sealed chamber, the diaphragm in the emergency brake will remain fully extended thereby compressing the large spring. However, should pressure fall, or should there be a leak in the sealed chamber, the diaphragm will be unable to hold the large compression spring in place. When this occurs, either slowly or quickly, the large compression spring will move the second pressure plate causing the push rod to be extended out thereby applying the brakes of the vehicle.
Under normal conditions, when the vehicle is parked, the air pressure to the emergency brake portion is cut off causing the large compression spring to apply the brakes.
In the transportation industry, it is becoming ever more desirable to provide more powerful spring brake actuators without changing their size. Increasing load sizes, new regulations and other factors have created a need for additional power in a spring brake with the same dimensional profile as existing double diaphragm spring brakes.
A stronger spring brake which takes up the same or a smaller space can result in great savings in the transportation industry. Under present regulations, a loaded truck must be able to apply its brakes and hold its position on a twenty percent (20%) grade. For many heavy vehicles, in order to accomplish this requires additional brake actuators and/or additional axles with brake actuators on them. With stronger brake actuators, fewer of them are needed to bring or hold such a vehicle at rest, thereby saving the cost of the additional brake actuators and/or additional axles.
There is also a need for a more powerful spring brake which fits into a smaller space. This need is driven by such factors as the installation of vehicle air suspensions, lowered floor heights, shorter wheel bases, and the addition of new and bulky chassis equipment. All of these factors compete for the same space occupied by the spring brake.
A spring brake assembly of smaller size which provides the same power as a larger assembly will also reduce weight and cost. A truck tractor and semi trailer may use 8 spring brake actuators on its axles. Replacing these with smaller units having the same strength that are two pounds lighter will reduce the weight by 16 pounds. While this may not seem significant at first blush, a liquid hauling vehicle is frequently loaded to the exact legal limit. Over the life of that vehicle, the 16 pound reduction will convert to thousands of dollars of hauling revenue.
Stronger brake assemblies deployed in the same space can improve the stopping characteristics of a vehicle thereby potentially increasing the gross vehicle weight allowance for the vehicle (i.e. more payload).
Existing service brake assemblies have been designed for attachment to the brake system of a heavy duty vehicle. The end of the service brake push rod is typically attached to a clevis which is, in turn, attached to the end of a slack adjuster arm located on a cam shaft which makes up part of the foundation brake of the vehicle. The push rod is moved in and out of the service brake assembly using pneumatic pressure as described above in order to operate the brakes of the vehicle. As this occurs, in some situations the push rod and clevis move the end of the slack adjuster through a slightly arcuate path around the cam.
For decades, the pressure plates used in existing diaphragm-based spring brake actuators have been relatively small in comparison to the overall profiles of these units. In a typical brake actuator, the pressure plate in the service brake chamber has approximately the same diameter as the pressure plate in the emergency brake chamber. The edges of such pressure plates have traditionally been restricted to the central portion of the brake chamber, presumably to allow sufficient space around the edges of the plates for the diaphragm to fold over itself. However, these traditional wide tolerances that have developed over time are far more than is necessary for the diaphragm to function properly, and have unnecessarily limited the sizes of the pressure plates used, and therefore unnecessarily inhibit the potential force that can be delivered to the push rod by the spring brake actuator.